The century saw a major shift in the way that vast numbers of people lived, as a result of changes in politics, ideology, economics, society, culture, science, technology, and medicine. Terms like ideology, world war, genocide, and nuclear war entered common usage. Scientific discoveries, such as the theory of relativity and quantum physics, drastically changed the worldview of scientists, causing them to realize that the universe was fantastically more complex than previously believed, and dashing the hopes at the end of the 19th century that the last few details of scientific knowledge were about to be filled in. Accelerating scientific understanding, more efficient communications, and faster transportation transformed the world in those hundred years more rapidly and widely than in any previous century. It was a century that started with horses, simple automobiles, and freighters but ended with luxury sedans, cruise ships, airliners and the space shuttle. Horses, Western society's basic form of personal transportation for thousands of years, were replaced by automobiles and buses within the span of a few decades. These developments were made possible by the large-scale exploitation of fossil fuel resources (especially petroleum), which offered large amounts of energy in an easily portable form, but also caused widespread concerns about pollution and long-term impact on the environment. Humans explored outer space for the first time, even taking their first footsteps on the Moon.

Mass media, telecommunications, and information technology (especially computers, paperback books, public education, and the Internet) made the world's knowledge more widely available to people. Many people's view of the world changed significantly as they became much more aware of the struggles of others and, as such, became increasingly concerned with human rights. Advancements in medical technology also improved the welfare of many people: the life expectancy of the world increased from 35 years to 65 years. Rapid technological advancements, however, also allowed warfare to reach unprecedented levels of destruction. World War II alone killed over 60 million people, while nuclear weapons gave humankind the means to annihilate or significantly harm itself in a very short period of time. The world also became more culturally homogenized than ever with developments in transportation and communications technology, popular music and other influences of Western culture, international corporations, and what was arguably a true global economy by the end of the century.

After gaining political rights in the United States and much of Europe in the first part of the century, and with the advent of new birth control techniques women became more independent throughout the century.

In classical music, composition branched out into many completely new domains, including dodecaphony, aleatoric and chance music, and minimalism. Electronic musical instruments were developed as well, vastly broadening the scope of sounds available to composers and performers.

Rock and Roll and Jazz styles of music are developed in the United States, and quickly become the dominant forms of popular music in America, and later, the world.

The automobile provided vastly increased transportation capabilities for the average member of Western societies in the early to mid-century, spreading even further later on. City design throughout most of the West became focused on transport via car. The car became a leading symbol of modern society, with styles of car suited to and symbolic of particular lifestyles.

Sports became an important part of society, becoming an activity not only for the privileged. Watching sports, later also on television, became a popular activity.

Culture and entertainment

As the century began, Paris was the artistic capital of the world, where both French and foreign writers, composers and visual artists gathered.

Movies, music and the media had a major influence on fashion and trends in all aspects of life. As many movies and much music originate from the United States, American culture spread rapidly over the world.

Tango was created in Argentina and became extremely popular in the rest of America and Europe. Blues and jazz music became popularized during the 1910s and 1920s in the United States. Blues went on to influence rock and roll in the 1950s, which only increased in popularity with the British Invasion of the mid-to-late 1960s. Rock soon branched into many different genres, including heavy metal, punk rock, and alternative rock and became the dominant genre of popular music. This was challenged with the rise of hip hop in the 1980s and 1990s. Other genres such as house, techno, reggae, and soul all developed during the latter half of the century and went through various periods of popularity.

The modern art movement revolutionized art and culture and set the stage for both Modernism and its counterpart postmodern art as well as other contemporary art practices.

Art Nouveau began as the most advanced architecture and design but went unfashionable after World War I. The style was very dynamic and highly inventive, however the depression of the Great War made it difficult to keep up such a high standard.

In Europe, modern architecture departed radically from the excessive decoration of the Victorian era. Streamlined forms inspired by machines became more commonplace, enabled by developments in building materials and technologies. Before World War II, many European architects moved to the United States, where modern architecture continued to develop.

The automobile vastly increased the mobility of people in the Western countries in the early-to-mid-century, and in many other places by the end of the 20th century. City design throughout most of the West became focused on transport via car.

The popularity of sport increased considerably—both as an activity for all, and as entertainment, particularly on television.